The advantages of HVDC components have been commercially exploited since 1954 when the first HVDC transmission was commissioned, the so-called HVDC Classic. Mercury-arc valves were eventually replaced with high power thyristors and DC transmissions have reached several GW, over +1-800 kV, and distances around 1000 kilometers. In 1997, a new breed of HVDC converter stations and HVDC transmissions were introduced, the so-called HVDC Light.
HVDC converter bridges and lines or cables can be arranged into a number of configurations for effective utilization. In a Back-to-Back configuration two HVDC converters are connected more or less directly to each other on the DC side, with the purpose of e.g. interconnecting two asynchronous AC power networks, or to regulate the flow of power in an AC power network. Back-to-back DC links are used in for example Japan, Brazil and Argentina for interconnections between power system networks of different frequencies (50 and 60 Hz).
The integral part of an HVDC power converter is the valve or valve arm. It may be non-controllable if constructed from one or more power diodes in series or controllable if constructed from one or more thyristors in series. FIG. 1a schematically shows the electric circuit network for a conventional six-pulse converter unit 5. The standard bridge or converter valve group 10 in HVDC Classic is defined as a double-way connection comprising six valves 20 or valve arms which are connected to one or more physical transformer units 30 as illustrated in FIG. 1a. Electric power flowing between the HVDC valve group and the AC system is three phase. When electric power flows into the DC valve group from the AC system then it is considered a rectifier. If power flows from the DC valve group into the AC system, it is an inverter. Each valve consists of many series connected thyristors in thyristor modules. FIG. 1a represents the electric circuit network depiction for the six pulse valve group configuration. FIG. 1b is the graphical symbol of a 6 pulse converter unit.
Today nearly all HVDC power converters with thyristor valves are assembled in a converter bridge of twelve pulse configuration. FIG. 2a demonstrates a twelve pulse converter with two three phase converter transformers 31, 32 with one DC side winding as an ungrounded star connection 31 and the other a delta configuration 32. Consequently the AC voltages applied to each six pulse valve group 10 which make up the twelve pulse valve group 40 have a phase difference of 30 degrees which is utilized to cancel the AC side 5th and 7th harmonic currents and DC side 6th harmonic voltage, thus resulting in a significant saving in harmonic filters. FIG. 2a also shows the outline 50 around each of the three groups of four valves in a single vertical stack. These are known as “quadrivalves” and are assembled as one valve structure by stacking four valves in series. Since the voltage rating of thyristors is several kV, a 500 kV quadrivalve may have hundreds of individual thyristors connected in series groups of valve or thyristor modules. FIG. 2b is the graphical symbol of a 12 pulse converter unit.
FIG. 3 is a scheme over a conventional back-to-back 12 pulse AC-AC converter, comprising an AC inlet 50a, an AC outlet 50b, two 12-pulse HVDC converter units 40 which are assumed to be arranged in a back-to-back configuration. The converter units are controlled by a control unit (not shown). Each converter unit comprises two six-pulse valve groups in series according to FIG. 2a. The inlet valve groups being connected to the AC inlet via separate inlet transformers 30a, and the outlet valve groups being connected to the AC outlet via separate outlet transformers 30b. 
The converter transformers of a 12-pulse HVDC converter setup which together form a transformer arrangement can take different configurations, see FIGS. 4a-d. The configurations shown therein comprise one single 12-pulse group, which is used as a basic building block for HVDC converter systems. The 12-pulse group can be applied from neutral to pole (single-pole configurations), with two 12-pulse groups to poles of different voltage polarity (bipolar configurations) or with the neutral point in the middle of the converter with mid-point grounding, leading to a six-pulse group between neutral and pole. The transformers used in the different configurations are three-phase three-winding transformers (FIG. 4a), three-phase two-winding transformers (FIG. 4b), single-phase three-winding transformers (FIG. 4c), and single-phase two-winding transformers (FIG. 4d).
HVDC systems are expensive that there is a desire to be able to choose the most cost efficient HVDC transformer configuration for a given HVDC system topology, taking into consideration the need of spare HVDC transformers.